1. Field of the Invention
The present invention relates generally to heat exchangers and, more specifically, to a stamped manifold and a method of making the same for a heat exchanger.
2. Discussion of the Related Art
Heat exchangers are commonly utilized in air conditioning systems such as those found in a motor vehicle. Heat exchangers, such as condensers, are typically manufactured in a “tube and fin” fashion. The heat exchanger typically includes a plurality of U-shaped tubes having a fluid passing therethrough and a plurality of fins extending between the tubes. The fins are attached to the U-shaped tubes to effectively increase the surface area of the tubes, thereby enhancing heat transfer capability of the tubes. A number of U-shaped tubes and fins may be stacked on top of each other, leaving a small opening to allow passage of air in between them, to create “condenser cores.”
The number of U-shaped tubes and fins depends on thermal capacity requirements of the heat exchanger. In order to connect these tubes together so that the fluid can flow through the tubes, manifolds are used having a series of openings or “mating” tubes corresponding to and mating with the ends of the U-shaped tubes. The manifolds have an inlet port and an outlet port which circulate the fluid through the heat exchanger and then return the fluid to a remote location for subsequent recycling. The condenser cores are typically rectangular in shape when assembled. The long U-shaped tubes, which are the foundation of condenser cores, are usually arranged so that the long-side of the tubes are horizontally arranged. Condensers are typically rectangular in shape, with one end of the condenser occupied by the manifold. Manifolds are long, pipe-like tanks with multiple mating tubes to attach to the open ends of the U-shaped tubes. Manifolds have distinct chambers inside to facilitate the flow of refrigerant. Manifolds are typically vertically arranged, so that the long end of the manifold is perpendicular to the ground. Manifolds typically have inlet and outlet ports, so that the refrigerant can be introduced from the rest of the air conditioner system to the manifold, and returned to the rest of the air conditioner system, once the refrigerant passes through the condenser.
There are many methods to manufacture manifolds, with several distinctive categories of methods. The first method is to entirely form the shape of a manifold through machining a solid piece of metal. This type of manufacturing process requires relatively low initial investment, typically utilizing computer numerical control (CNC) machining equipment. However, CNC machining can be cost detrimental for mass production of the manifolds due primarily to the amount of time it takes to machine a manifold. Since an entire shape of the manifold has to be cut out from a block of material, it may take hours to machine a manifold even with a very fast CNC machine. Therefore, this type of manufacturing method is not very well suited for high volume production, which is typically required for an automotive application.
The second method of manufacture is to utilize a cold-forging process, or a similar process called a back-extrusion process. Compared to the machining process, this type of manufacturing method requires far more initial investment. Firstly, cold-forging equipment or back-extrusion equipment must be acquired. Secondly, a dedicated die has to be manufactured to form the desired shape, in this case a shape of a manifold. Although productivity of cold-forged/back-extruded manifolds are far superior to manifolds manufactured by a machining process, there are several drawbacks. A cold-forged/back-extruded process requires a general shape to be formed via an extrusion process. The extrusion process creates a general outline of the manifold, typically out of an ingot of aluminum, thereby eliminating unnecessary machining. The extruded material is then machined to further shape the material to prep for the cold forging process. The machined shape is then cold-forged/back-extruded to form the detailed shape of the manifold. In particular, individual mating tubes on the manifolds that act as a receptacle to the individual U-shaped tubes are complex in shape. These mating tubes must first be machined to form solid, individual tubes not having the finished shape of a thin wall and openings to the inner manifold. This machined shape is then cold-forged/back-extruded to form the final shape with the thin wall and an opening to the inner side of the manifold (the mating tubes are not completely machined in the machining process, since doing so would be time-consuming and cost prohibitive). Although the cold-forging/back-extruded process is far more cost effective than machining, there are multiple manufacturing steps involved thereby increasing the manufacturing cost.
The third method of manufacture is a stamping process. In this approach, a manifold is manufactured out of generally flat sheet of metal, with the finished manifold consisting of either one sheet of metal or multiple sheets of metal. The quantity of sheet metal varying depending on different methods of manufacture. Forming the shape of the manifold is generally accomplished through the metal stamping process, using a die and a stamping press.
Generally, all stamped manifolds will have a few things in common. First, the individual mating tubes are shaped via a drawing process. Secondly, each end region of the sheet metal is bent into shape, so that each end region of the sheet metal forms two distinct tanks. Thirdly, all stamped manifolds are either soldered or brazed together, using either a cladded aluminum sheet metal (aluminum sheet metal with at least 2 layers of different aluminum alloys with varying liquefying temperature), or a standard sheet metal in combination with brazing sheet/soldering paste. Various examples of prior art embodiments are focused on the means of treating the two ends of the sheet metal to form the two separate chambers within the manifold.
A first prior art example of a stamped manifold is described in Dawson, U.S. Pat. No. 4,770,240. In this patent, sheet metal material in between a pair of mating tubes is bent away from the tubes to form a shape resembling an inverted “V”. Each end of the sheet metal is then bent to form two separate tanks, with each end of the sheet metal bent to rest along the exterior surface of the inverted “V” shape. There are certain drawbacks to this design as discussed below. First, and foremost, in order to form the finished manifolds, extremely high heat is applied to the entire structure. When extremely high heat is applied to a material, especially a thin sheet of material, the material is prone to movement. Each end of the sheet metal simply rests against the inverted “V.” In such a configuration, it is highly likely that the end of the bent sheet, along the entire length of the manifold, may not remain in position during the brazing/soldering process. This movement leads to areas that are not properly brazed/soldered. Since manifolds must be leak-free, such manifolds with void areas are useless. Therefore, this configuration may result in a high defect rate, leading to low productivity and high cost.
A second prior art example of a stamped manifold is described in Dawson, U.S. Pat. No. 5,163,509. The manifold as illustrated in this patent is constructed out of not a single sheet of metal, but two separate sheets of metal, with each sheet metal forming an individual manifold tank. Therefore, instead of having a single sheet of metal formed to create two individual tanks, the individual tanks are individually manufactured. The tanks are then positioned side-by-side to take the shape of the complete manifold. The two tanks have only one row of mating tubes individually, so that when the tanks are paired up they have the requisite pair of mating tubes to be attached to the U-shaped tubes. Although this embodiment resolves the issue of the brazing problem as discussed above with respect to Dawson U.S. Pat. No. 4,770,240, the cost to manufacture is higher than other embodiments of a stamped manifold.
The third embodiment of a stamped manifold is illustrated in Rhodes, U.S. Pat. No. 6,216,777 B1. In this patent, the approach to creating the two tanks is through the bending of the two ends of the sheet metal to form the individual tanks, which is similar to Dawson U.S. Pat. No. 4,770,240. However, in this case, the two ends of the sheet metal are not simply resting against the exterior surface of a bent inverted “V” portion of the sheet metal surface as in Dawson U.S. Pat. No. 4,770,240. Instead, a single central channel is created running the entire length of the manifold to act as a receptacle of the two ends of the sheet metal. By having a single central channel, this embodiment attempts to resolve the issue faced in Dawson U.S. Pat. No. 4,770,240, i.e., a relatively high likelihood of a brazing failure, due to movement of material when exposed to heat during brazing/soldering. Although Rhodes U.S. Pat. No. 6,216,777 B1 embodiment is a more robust design compared to Dawson U.S. Pat. No. 4,770,240, there are several shortcomings.
By having only a single channel to receive the two ends of the sheet metal, there is a potential for one end of the sheet metal to dominate the channel, forcing the other end of the sheet metal to be positioned out of the channel. This can occur for a portion of the channel or the entire length of the manifold. If either end of the sheet metal is not properly installed into the single channel several possible failure modes may occur. A first failure mode occurs when one end of the sheet metal is significantly off the single channel, thereby preventing the manifold tanks from properly sealing during the brazing/soldering process. If this occurs, the manifold may be useless, since manifolds have to be leak-proof. A second failure mode occurs when the end of the sheet metal is slightly off the single channel, thereby creating a passageway between the two tanks. If this type of failure occurs, it may be an extremely difficult failure to detect. The failure may only be detectable due to the fact that the failure prevents the manifold from operating properly when the defective manifold is assembled into a condenser and tested as a unit.
It is also possible that for a certain portion of the single channel length, the two ends of the sheet metal may float off the channel, creating a void between the two tanks. This type of a failure may be likely, since two separate parts must fit in a single space. Considering that the stamping presses are often run at a high speed to optimize production, the likelihood for this type of failure increases. This type of a failure is also problematic, since it is very difficult to inspect for this type of a failure once the manifold takes its final shape. Often, it is only detectable once the manifold is assembled onto a condenser and the air conditioner is operated for the first time.